Low impedance helical delay line



NOV. 1970 J. SINGLETARY, JR

LOW IMPEDANCE HELICAL DELAY LINE Filed Sept. 20, 1967 (PRIOR ART) INVENTOR. JUNE SINGLETARY Jr.

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A T TORNE Y United States Patent US. Cl. 333-31 9 Claims ABSTRACT OF THE DISCLOSURE A low impedance helical delay line is disclosed having an improved delay performance due to the elimination of undesirable air gaps between the hollow high permittivity dielectric cylinder and the helix which it surrounds, and between the outer conductive layer and the high permittivity dielectric cylinder.

BACKGROUND OF THE INVENTION Delay at radio frequencies is usually provided by long lengths of line or by transformation to another mode of propagation, e.g., the acoustic mode or the magnetic mode used in ultrasonic and magnetostrictive delay lines, respectively. Such conventional delay lines frequently have one or more serious disadvantage such as large size, narrow frequency range or very high loss. The high loss and narrow frequency range are often a result of the transducers which are required to change the mode of propagation. At present less than one percent of the energy passing twice through a transducer can be conserved.

To provide a delay line which did not require a transformation from the electromagnetic mode and which would provide an increased delay per unit of length, the inductance of a coaxial line was increased by replacing the center conductor with a helix. Such a delay line is described by J. Millman and H. Taub in Pulse, Digital and Switching Wave Forms, New York: McGraw-Hill 1965, pp. 798-799. However, since the characteristic impedance of a delay line is proportional to the square root of the inductance, the increase in delay provided by the helical delay line is accompanied by an increase in characteristic impedance. The high impedance helical delay line disclosed in the Millman et al. publication consists of a helix which is wound on an inner core which may consist of magnetic material. A polyethylene dielectric material is situated between the helix and the outer conductor.

In an effort to provide a helical delay line having a characteristic impedance which can be maintained at a low value compatable with the transmission system in which it is used, a helical delay line consisting of a center conductor wound on a ferrimagnetic core and embedded in a high permittivity dielectric was developed. This configuration increased the inductance and the capacitance per unit of length. The delay, which is proportional to the square root of the product of inductance and capacitance is thereby increased, while the impedance, which is proportional to the square root of the ratio of inductance to capacitance, can be maintained at a low value.

In conventional high impedance delay lines such as those described in the Millman et al. publication, the incidental air gaps between the dielectric and outer conductor or between the dielectric and the helix are of little consequence. However, these air gaps are of paramount importance in low impedance delay lines using a high permittivity dielectric material between the helix and the outer conductor. Low impedance delay lines are conventionally made by applying a helical conductor to a cylindrical ferrimagnetic rod and then inserting the rod into a hollow, high permittivity dielectric cylinder which is enclosed by an outer conducting tube of metal. Even when an attempt is made to control the tolerances so that the helical conductor fits tightly within the dielectric cylinder 21 small air gap exists between these two members.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a helical delay line in which the undesirable air gaps between the high permittivity dielectric cylinder and the helical and outer conductors are eliminated.

Another object of this invention is to provide an improved helical delay line having a low characteristic impedance and an increased delay per unit of length.

A further object of this invention is to provide an improved helical delay line which exhibits a decrease in attenuation per unit of delay time.

Briefly, the helical delay line of this invention comprises a'helical conductor surrounding a core of magnetic material, a hollow dielectric cylinder surrounding the helical conductor and an outer cylindrical conductor surrounding the hollow dielectric cylinder. This invention is characterized by the location of the helical conductor on the inner surface of the hollow dielectric cylinder. Furthermore, the outer cylindrical conductor of the helical delay line consists of a metallic film deposited on the,

outer surface of the hollow dielectric cylinder.

Other objects, features and advantages of this invention will become apparent during the course of the following detailed description and the attached drawings, on which, by way of example, only the preferred embodiment of this invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a prior art delay line illustrating the existance therein of undesirable air gaps,

FIG. 2 is a cross-sectional view of a preferred embodiment of this invention, and

FIG. 3 is a fragmental cross-sectional view of another embodiment of this invention.

DETAILED DESCRIPTION FIG. 1 shows a cross section of a conventional low impedance helical delay line 10 which consists of a ferrite core 11 on which a helix 12 is Wound. A hollow, high permittivity dielectric cylinder 12, which surrounds the helix 12, is surrounded by an outer metallic tube 14. Such conventionally made low impedance delay lines are constructed by applying the helical conductor 12 to the cylindrical rod 11 and then inserting the rod into the hollow, high permittivity dielectric cylinder 13. The cylinder 13 is then inserted into the outer metallic tube 14. Even when these components are made to closely controlled tolerances, an air gap 16-exists between the helix 12 and the dielectric cylinder 13, and an air gap 17 exists between the cylinder 13 and the metallic tube 14.

It was found that delay lines constructed in accordance with FIG. 1 produced significantly less delay than the amount which is theoretically obtainable. An analysis of the effect of the small air gaps 16 and 17 indicated that the delay time drops by the factor where:

r =the reduced delay due to the presence of air gaps, i =the delay which can be realized in the absence of air gaps, e l permittivity of the dielectric cylinder 13,

=inside diameter of the metallic tube 14, b=diameter of the helix 12,

a=air gap width between the helix 12 and the inside surface of the'dielectric cylinder 13, and

-c=air gap width between the outside surface of the dielectric cylinder 13 and the inside surface of metallic tube 14.

TABLE 1 Ratio of is to ta nominal With ey=200 With e, =900 Width of air gaps a and e l. 0 1. 0 0.28X- in- 0. 75 0. 46 0.5)(10- in 0. 60 0. 33 1.0X10- in 0. 47 0. 24 2.0X10' in 0.38 0. 19

Table I illustrates the serious effect that air gaps have on delay time. The outer air gap 17 can be avoided by metallizing the outer surface of the dielectric cylinder 13. The deteriorating effect of the inner air gap 16 is primarily a result of its effect on the capacitance of the delay line. This effect can be eliminated by forming the helix on the inside of the dielectric material as shown in FIG. 2. When this is done, the small air gap inside the helix is actually of some benefit if a conventional ferrite of cubic crystalline structure is used as the magnetic material. Such ferrites generally have a dielectric Q of about unity, and the air gap inside the helix reduces ferrite dielectric loss.

Referring to FIG. 2, there is shown a cross-sectional view of a preferred embodiment of this invention. In this embodiment a helix 21 is adherently located on the inside wall of the hollow dielectric cylinder 22. The helix may be formed by machining a helical groove inside the dielectric cylinder, completely metallizing the inside of the cylinder 22, and thereafter polishing the excess metal so that only the helical pattern remains. The helix could also be formed by a photoresist technique such as that disclosed in my copending patent application entitled Fabrication of Patterns on Nonplanar Surfacesjfiled on even date herewith. As shown in FIG. 3, wherein components similar to those of FIG. 2 are designated by primed reference numerals, a helix '21 constructed in accordance with my copending application would lie on the inner surface of the dielectric cylinder 21' and extend inwardly therefrom. A ferrite rod 23, the length of which is slightly longer than the cylinder 22, is located inside the cylinder 22 so that the ends thereof project a short distance from the end of the cylinder 22. Part of the rod 23 is cut away to more clearly illustrate the location of the helix 21 on the inside surface of the cylinder 22. The ends of the ferrite rod are metallized at portions 24 and 25 so that a conductive path is formed from each end of the ferrite rod to the respective end of the helix 21. The widths of the metallized portions 24 and 25 are nonuniform to provide a better match between the helix 21 and the center coaxial conductor to which it will be connected. The metallized portions 24 and 25 are electrically connected to each end of the helix by drops of conductive epoxy 31 and 32, respectively. The outside surface of the dielectric cylinder 22 is metallized to form a metallic film 26.

The c linder 22 may be made from any low loss dielectric material having a high dielectric constant. The rod 23 may consist of a ferrite of the hexagonal crystalline structure (ferroxplana).

The delay line shown in FIG. 2 may be connected to a coaxial cable by electrically connecting the metallized portions 24 and 25 to the inner coax conductors while connecting the outer coax conductors to the metalized portion 26. If desired, an impedance matching section can be affixed to each end of the delay line. Such a section has not been described since it does not form a part of this invention. A matching section could be formed by extending and tapering the ends of the ferrite rod 23. The metallized portions 24 and 25 could be replaced by a helix, the width of which gradually increases as it extends away from the delay line helix while the pitch thereof decreases.

I claim:

1. A helical delay line comprising:

a hollow dielectric cylinder,

a helix adherently located on the inner surface of said hollow dielectric cylinder,

a conductive coating on the outer surface of said hollow dielectric cylinder,

a core of magnetic material disposed within said hollow dielectric cylinder and spaced from said helix,

a metallied portion at each end of said core, and

conductive means connecting each end of said helix to the metallied portion adjacent thereto.

2. A helical delay line in accordance with claim 1 wherein said outer cylindrical conductor consists of a metallic film on said dielectric cylinder.

3. A helical delay line in accordance with claim 2 wherein said core consists of a ferrite of cubic crystalline structure.

4. A helical delay line in accordance with claim 3 wherein said hollow dielectric cylinder consists of a high permittivity material.

5. A helical delay line as defined in claim 4 wherein the inner surface of said hollow dielectric cylinder is continuous and the outer surface of said helix is adherently affixed to the inner surface of said dielectric cylinder, the inner surface of said helix extending inwardly from said dielectric cylinder toward said magnetic core.

6. A helical delay in accordancewith claim 1 wherein said hollow dielectric cylinder contains a helical groove along its inner surface and said helical conductor is situated in said groove so that the inner surface of said helical conductor is substantially flush with the inner surface of said dielectric cylinder.

. 7. A helical delay line comprising:

a core of magnetic material,

a hollow dielectric cylinder surrounding said core, said cylinder being shorter than said core so that the ends of said core project from the ends of said cylinder,

a conductive helix surrounding said core, said helix being characterized in that it is adherently located on the inner surface of said hollow dielectric cylinder,

insulating means for preventing said core from shorting adjacent turns of said helix,

an outer cylindrical conductor surrounding said hollow dielectric cylinder, and

conductive means at each end of said delay line for connecting said helix to the inner conductor of a coaxial cable, said conductive means comprising a metallized portion surrounding each end of said core and extending along the surface of said core to a location adjacent to the respective end of said helix, and a conductive connection between each end of said helix and the metallized portion adjacent thereto.

8. A helical delay line in accordance with claim 7 wherein the widths of said metallied portions are nonuniform.

9. A helical delay line in accordance with claim 7 wherein said conductive connection comprises conductive epoxy.

References Cited UNITED STATES PATENTS 2,115,826 5/1938 Norton et al. 333-31 2,387,783 10/ 1945 Tawney 333-31 (Other references on following page) References Cited 2,520,991 9/1950 Yolles 333-31 X 2,619,537 11/1952 Kihn 333-31 2,787,656 4/ 1957 Raisbeck 178-45 2,838,735 6/1958 Davis 333-3 1 5 3,238,477 3/ 1966 Brueckmann 333-96 OTHER REFERENCES Kirschbaum, Herbert S.: The Characteristics Impedance and Phase Velocity of a Shielded Helical Trans- 10 mission Line, Communications & Electronics (AIEE), September 1959, pp. 444-450.

HERMAN KARL SAALBACH, Primary Examiner W. N. PUNTER, Assistant Examiner US. Cl. X.R. 333-95, 96 

